Method of Monitoring Dislodgement of Venous Needles in Dialysis Patients

ABSTRACT

A method of detecting a dislodged needle in a hemodialysis procedure includes measuring venous drip pressure of the dialysis machine of a patient undergoing hemodialysis, analyzing the venous drip pressure and deriving intravascular blood pressure at a location of venous needle insertion into the patient, comparing the derived intravascular blood pressure to a standard, repeating the measuring, analyzing and deriving, and comparing steps and, if the intravascular blood pressure is within a specified range of the standard, determining that a needle has been dislodged in the hemodialysis procedure. A method of alerting the patient and medical personnel of a dislodged needle in a hemodialysis procedure includes detecting a drop in intravascular pressure derived from measured venous drip pressure, determining that a needle is dislodged, and alerting medical personnel of the dislodged needle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.10/516,389 filed Jul. 29, 2002, now U.S. Pat. No. 7,597,666, which isincorporated herein by reference, which is a 371 of PCT/US02/23958 filedJul. 29, 2002 which, in turn, claims the benefit of U.S. provisionalapplication Ser. No. 60/308,872 filed Jul. 30, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to devices and methods for detectingfailure in dialysis systems.

2. Background Art

Proper functioning of the vascular system is essential for the healthand fitness of living organisms. The vascular system carries essentialnutrients and blood gases to all living tissues and removes wasteproducts for excretion. The vasculature is divided into differentregions depending on the organ systems served. If vessels feeding aspecific organ or group of organs are compromised, the organs andtissues supplied by those vessels are deleteriously affected and caneven fail completely.

Vessels, especially various types of arteries, not only transmit fluidto various locations, but are also active in responding to pressurechanges during the cardiac cycle. With each contraction of the leftventricle of the heart during systole, blood is pumped through the aortaand then distributed throughout the body. Many arteries contain elasticmembranes in their walls that assist in expansion of the vessel duringsystole. These elastic membranes also function in smoothing pulsatileblood flow throughout the vascular system. The vessel walls of sucharteries often rebound following passage of the systolic pressurewaveform.

In autoregulation, cerebral blood vessels maintain constant cerebralblood flow by either constricting or dilating over a certain meanarterial blood pressure range so that constant oxygen delivery ismaintained to the brain. Vascular failure occurs when the pressure dropstoo low and the oxygen delivery starts to fall. If the blood pressuregets too high and the vessels can no longer constrict to limit flow,then hyperemia breakthrough or loss of autoregulation can occur. Both ofthese conditions are pathologic states, and have been described in theliterature in terms of mean arterial pressure and cerebral blood flowvelocity, but there are others that cannot be explained based on thatmodel. The failure of the model is that it relies upon systemic bloodpressure. The pressure of blood in the brain itself is not beingmeasured directly. The resultant pressure curve has an S-shaped curve.

The force applied to the blood from each heartbeat is what drives theblood forward. In physics, force is equivalent to mass timesacceleration. But when blood is examined on a beat-to-beat variation,each heartbeat delivers about the same mass of blood, unless there issevere loss of blood or a very irregular heart rhythm. Therefore, as afirst approximation, the force of flow on the blood at that particularmoment is directly proportional to its acceleration.

Diseased blood vessels lose the ability to stretch. The elasticity orstretch of the blood vessel is very critical to maintaining pulsatileflow. When a muscle is stretched, it is not a passive relaxation. Thereis a chemical reaction that happens within the muscle itself that causesa micro-contracture to increase the constriction, so that when a bolusof blood comes through with each heartbeat, it stretches the bloodvessel wall, but the blood vessel then contracts back and gives the kickforward to maintain flow over such a large surface area. This generatesa ripple of waves, starting in the large vessel of the aorta and workingits way through the rest of the vessels. As vessels become diseased,they lose the ability to maintain this type of pulsatile flow.

Further, if vessels are compromised due to various factors such asnarrowing or stenosis of the vessel lumen, blood flow becomes abnormal.If narrowing of a vessel is extensive, turbulent flow can occur at thestenosis resulting in damage to the vessel. In addition, blood cannotflow adequately past the point of stenosis, thereby injuring tissuesdistal to the stenosis. While such vascular injuries can occur anywherethroughout the body, the coronary and cerebral vascular beds are ofsupreme importance for survival and well-being of the organism. Forexample, narrowing of the coronary vessels supplying the heart candecrease cardiovascular function and decrease blood flow to themyocardium, leading to a heart attack. Such episodes can result insignificant reduction in cardiac function and death.

Abnormalities in the cerebral vessels can prevent adequate blood flow toneural tissue, resulting in transient ischemic attacks (TIAs),migraines, and stroke. The blood vessels that supply the brain arederived from the internal carotid arteries and the vertebral arteries.These vessels and their branches anastomose through the great arterialcircle, also known as the Circle of Willis. From this Circle arise theanterior, middle and posterior cerebral arteries. Other arteries such asthe anterior communicating artery and the posterior communicating arteryprovide routes of collateral flow through the great arterial circle. Thevertebral arteries join to form the basilar artery, which itselfsupplies arterial branches to the cerebellum, brain stem and other brainregions. A blockage of blood flow within the anterior cerebral artery,the posterior cerebral artery, the middle cerebral artery, or any of theother arteries distal to the great arterior circle results incompromised blood flow to the neural tissue supplied by that artery.Since neural tissue cannot survive without normal, constant levels ofglucose and oxygen within the blood and provided to neurons by glialcells, blockage of blood flow in any of these vessels leads to death ofthe nervous tissue supplied by that vessel.

Strokes result from blockage of blood flow in cerebral vessels due toconstriction of the vessel resulting from an embolus or stenosis.Strokes can also arise from tearing of the vessel wall due to any numberof circumstances. Accordingly, a blockage can result in ischemic strokedepriving neural tissue distal to the blockage of oxygen and glucose. Atearing or rupture of the vessel can result in bleeding into the brain,also known as a hemorrhagic stroke. Intracranial bleeding exertsdeleterious effects on surrounding tissue due to increased intracranialpressure and direct exposure of neurons to blood. Regardless of thecause, stroke is a major cause of illness and death. Stroke is theleading cause of death in women and kills more women than breast cancer.

Currently, more than three-quarters of a million people in the UnitedStates experience a stroke each year, and more than twenty-five percentof these individuals die. Approximately one-third of individualssuffering their first stroke die within the following year. Furthermore,about one-third of all survivors of a first stroke experience additionalstrokes within the next three years.

In addition to its terminal aspect, stroke is a leading cause ofdisability in the adult population. Such disability can lead topermanent impairment and decreased function in any part of the body.Paralysis of various muscle groups innervated by neurons affected by thestroke can lead to confinement to a wheelchair, and muscular plasticityand rigidity. Strokes can leave many patients with little or no abilityto communicate either orally or by written means. Often, stroke patientsare unable to think clearly and have difficulties naming objects,interacting well with other individuals, and generally functioningwithin society.

Despite the tremendous risk of stroke, there are presently no convenientand accurate methods to access vascular health. Many methods rely oninvasive procedures, such as arteriograms, to determine whether vascularstenosis is occurring. These invasive techniques are often not ordereduntil the patient becomes symptomatic. For example, carotid arteriogramscan be ordered following a physical examination pursuant to theappearance of a clinical symptom. Performing an arteriogram is notwithout risks due to the introduction of dye materials into the vascularsystem that can cause allergic responses. Arteriograms also usecatheters that can damage the vascular wall and dislodge intraluminalplaque, which can cause an embolic stroke at a downstream site. It wouldtherefore be useful to develop a noninvasive or limited invasiveprocedure for assessing vascular health.

Further, in the field of hemodialysis and other techniques where bloodis removed from a patient for processing and then returned, it isimportant to periodically assess the blood flow rate through anarteriovenous fistula, graft, or catheter to monitor the onset ofstenosis. This is often accomplished by the reading of access pressuresthrough the venous and arterial access needles. Early detection ofstenosis associated with the placement of a fistula, graft, implantableport, or a catheter can permit low cost repairs to be made. On the otherhand, if these problems are ignored or not detected, the cost of therevision or replacement of the fistula, graft, implantable port, orcatheter can be very high and burdensome to the patient.

There have been several devices that have been developed to determinepressure inside a dialysis machine or during hemodialysis. For example,as disclosed in U.S. Pat. No. 5,454,374 to Omachi, access pressures canbe determined through volumetric manipulations involving thedetermination of a pressure head height of blood in a visual manner. Theblood line going to the dialysis machine is used to measure pressure andthe problem is one of determining the height between the transducer andthe patient's access site.

U.S. Pat. No. 4,710,163 to Levin et al. discloses a method and systemfor continuously monitoring patient heart rate and mean arterial bloodpressure during hemodialysis and for automatically controlling fluidextraction rate and/or dialysate sodium concentration in the event thatblood pressure and/or heart rate indicate onset or impending onset of apatient hypotensive episode. There are three separate machines forperforming these functions: an automated blood pressure monitor, anautomated patient heart rate monitor, and the hemodialysis machine. Theblood pressure monitor is essentially a device for measuring bloodpressure based on the blood in the patient's arm, i.e. a cuff thatinflates and deflates automatically to read the diastolic and systolicblood pressure readings. This device merely takes the place of an actualtechnician to take a blood pressure reading. The blood pressure readingsare derived from a standard blood pressure cuff on the patient's arm andnot from the intravascular blood near the access site for anextracorporeal circuit.

U.S. Pat. No. 6,623,443 to Polaschegg discloses a device that measuresand compares the amplitude of pressure pulses within an extracorporealcircuit to determine whether a stenosis has occurred therein. Thepeak-to-peak amplitude of the pressure waves created by variations inthe patient's blood pressure and variations in pressure created by theextracorporeal blood pump are used to indicate the presence of anobstruction in the circuit. A deviation in the peak-to-peak amplitude ofthe pressure signal from a predetermined standard value indicates astenosis or loss of occlusion of the roller pump. No standard is definedto indicate a stenosis that represents a significant risk to thepatient. No measurements or calculations of intravascular blood pressureoccur.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a dialysis circuit used to determine the relationshipbetween blood flow and hemodialysis machine venous drip chamber pressurewith hematocrit varied from 38.4% to 18.2%;

FIG. 2 shows the venous drip chamber pressure versus blood flow in ahemodialysis machine blood circuit for a range of hematocrit values,including a single curve showing venous needle pressure at a hematocritof 29.1%, wherein venous needle pressure is 0 mmHg when Qb=0 because thetransducer and the venous needle are at the same height, and venous dripchamber pressure is approximately −17 mmHg when Qb=0 because the venousneedle is 17 centimeters below the height of the drip chambertransducer;

FIG. 3 shows the receiver-operating characteristic (ROC) curves for theJanuary 1999 VAPRT for grafts (117) and fistulas (23) combined andgrafts alone, an area of 1 represents an ideal test, an area of 0.5indicates the test has only a 50% probability determining the correctoutcome, and an area from 0.80 to 0.90 implies a good test;

FIG. 4 shows the distribution of access pressure ratio values within thefour possible test groups: true positive, true negative, false positive,and false negative for patients with grafts;

FIG. 5 shows the access pressure ratio test results for three separatemonths of testing, wherein patients were followed for six months aftereach test for an access failure event;

FIG. 6 is a graph showing the relationship between coefficient B in theequation for venous drip chamber pressure with zero venous accesspressure VDP₀=0.00042329*Qb²+B*Qb 17.325 and hematocrit (Hct);

FIG. 7 is a flow chart depicting the inner workings of a deviceaccording to an aspect of the present invention;

FIG. 8 is a photograph of a percutaneous transluminal angioplasty;

FIGS. 9A and B are photographs depicting dialysis machines for use inconjunction with the device in accordance with an aspect of the presentinvention; and

FIG. 10 is a flowchart depicting a method according to an aspect of thepresent invention of detecting a dislodged needle during hemodialysis.

DETAILED DESCRIPTION OF THE INVENTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale, andsome features may be exaggerated or minimized to show details ofparticular components. Therefore, specific structural and functionaldetails disclosed herein are not to be interpreted as limiting, butmerely as a representative basis for teaching one skilled in the art tovariously employ the present invention.

Generally, according to an aspect of the present invention, a detectiondevice and method are provided for detecting variations in intravascularpressure that indicate irregular blood flow, i.e. a suspected blood flowrestriction or other blood flow problem, especially when a needle of ahemodialysis device has become dislodged from a patient. The deviceincludes an analyzer for automatically analyzing intravascular pressureupstream of the suspected location of irregular blood flow and comparingthe intravascular pressure to a standard, whereby variations in theintravascular pressure during multiple tests is indicative of a bloodflow restriction.

U.S. Pat. No. 7,597,666 to Frinak et al. disclosed for the first time amethod of detecting an irregular intravascular pressure by measuringextracorporeal pressure taken from a patient and analyzing theextracorporeal pressure with an algorithm to determine intravascularpressure. The intravascular pressure is compared to a standard in orderto determine if the patient is at risk of developing a stenosis.Variation of the calculated intravascular pressure multiple times withthe standard indicates irregular blood flow and risk of stenosis.

Dialysis is a very complicated procedure that must be carried out by ateam of trained professionals who are responsible for delivering safeand effective care to the patient. It can also be self-administered by apatient in their home, but only after the patient has undergoneextensive training. There are many ways that complications can ariseduring a dialysis session. Many of these potential issues areconstrained by alarm circuits and other safeguards built into thedialysis machine.

Hemodialysis machines utilize two needles, one to remove blood from thepatient (arterial) and one to put the dialyzed blood back into thepatient (venous needle). The venous needle can become dislodged from thepatient, such as accidentally pulled out of the access, which thenallows the blood being pumped back into the patient to run onto thefloor. Because of the relatively high blood flows of the dialysismachines (300 to 500 ml of blood per minute), if this dislodgement goesunnoticed the patient can bleed to death in a short amount of time. Forexample, an average male patient can lose 40% of their blood supply in 8minutes. Even in a hospital or clinical setting, dislodgement cansometimes occur without any visual detection by a medical staff becausea blanket can cover the bloodlines. This issue is even more of a concernwhen a patient is dialyzed overnight. This can be more convenient forpatients who do not want to spend the day in the hospital, with thehemodialysis procedure performed while they are asleep. However,overnight dialysis poses even more of a risk that the dislodgement ofthe venous line needle during the procedure will go unnoticed. Forexample, if the patient rolls over during sleep or otherwisesignificantly moves in the hospital bed, this can cause needledislodgement. A large quantity of blood can be lost and death can resultin many cases. It has been estimated that between 40 and 136 patientsdie each year in the US due to losing sufficient blood because of needledisplacement.

The current method of detecting dislodgement of a needle is visualmonitoring by staff that must instruct the patient not to cover venouslines with a blanket. While many hemodialysis machines do include somesort of alarm to indicate pressure changes in the venous and arterialbloodlines, dislodgement of needles generally do not trigger an alarm,so the dislodgment is often not detected until too late. The reason forthis is that small gauge needles that are used to minimize pain topatients create back-pressures that continue to be detected by themachine when the needle is dislodged. This sufficient back-pressurecreated in the tubing and needle masks the pressure drop at the tip ofthe needle if it becomes dislodged, such that the drop in the pressurecaused by the removal of the needle from the arm, and hence the loss ofthe pressure required to push the blood into the patient's arm, is nothigh enough to show a significant change in the pressure as measured bythe venous drip chamber transducer, especially if the range of alarm isnot set correctly on the machine. Thus, sufficient pressure remains inthe circuit between the tubing and the needle so that the measuredvenous drip pressure does not drop significantly, and no alarm is setoff. There is a need for a more reliable method of detectingdislodgement of venous needles from a patient as well as an alarm systemto turn off the blood pump on the dialysis machine and alert medicalpersonnel in time to save a patient's life.

According to an aspect of the present invention, a method is providedfor detecting a dislodged needle in a hemodialysis procedure bymeasuring venous drip pressure in a patient, analyzing the venous drippressure and deriving intravascular blood pressure at the location ofneedle placement in the patient. The actual pressure may be calculatedas seen at the tip of the venous needle, which when dislodged,dramatically decreases to zero or near zero. Hence, the radical changein this calculated pressure when a needle is dislodged allows for thedetermination that something is wrong with the venous needle and shouldbe investigated. According to another aspect of the present invention, amethod is provided of shutting down the dialysis machine and alertingmedical personnel of a dislodged needle in a hemodialysis procedure.

The “detection device” as disclosed herein is intended to include, butis not limited to, any device that is able to detect variations inintravascular pressure that indicate irregular blood flow. In oneembodiment, the intravascular pressure is venous pressure that isupstream of the suspected area or location of a blood flow restriction.An example of such a device is a hemodialysis machine.

The “analyzer device” as used herein is intended to include a devicethat is capable of automatically analyzing the intravascular pressure.Such an analyzer device can be computer-driven. For example, theanalyzer can include a device that is associated with a hemodialysismachine, such that it automatically assesses intravascular pressureduring hemodialysis. The analyzer can then equate and compare theintravascular pressure to a standard. An equation is used that estimatespressure inside a blood access site and is then used to detect irregularblood flow. In one embodiment, this equation is an algorithm thatcalculates the ratio between venous blood pressure and mean arterialpressure.

The term “variation” is intended to include an increase or decrease inthe derived intravascular pressure. Any deviation from the standard canbe indicative of a problem. Depending upon whether there is an increaseor decrease in intravascular pressure, the detection of the deviationhelps determine what the problem is at the access site. For example, ifthere is an increase in intravascular pressure, the problem potentiallyis something that blocks normal blood flow downstream of the measurementsite. The blockage represents a narrowing of a blood vessel thatincreases the risk for an access failure, a stroke, or a heart attack.If there is a decrease in intravascular pressure, this is indicative ofa blockage of normal blood flow upstream of the measurement site.

The term “communication device” as used herein is intended to include adevice operably connected to the detecting device for communicating awarning when the detecting device indicates an irregularity of bloodpressure of at least two uses of said device. The communicating devicecan be selected from, but is not limited to, electronic communications,a facsimile, a telephone, a cable modem, and a T1 connection.

The term “algorithm” as used herein is intended to encompass anycomputation that enables an individual to ascertain the informationnecessary for detecting irregular intravascular pressure. In oneembodiment, the algorithm is computer driven and follows the generalfunction shown in FIGS. 7A through D. The algorithm can be used as partof an integrated circuit. This circuit enables the algorithm to be moreeasily incorporated into a dialysis machine. The circuit can be createdusing technology known to those with skill in the art.

The method in accordance with the present invention may be practicedwith the following device. The device includes a detection device fordetecting irregular intravascular pressure, the device including ananalyzer for automatically monitoring intravascular pressure upstream ofthe suspected location of irregular blood flow, and a device forcomparing intravascular pressure to a standard, whereby variation in theintravascular pressure during multiple tests is indicative of irregularblood flow. As disclosed above, the device may be affixed to ahemodialysis machine; however, the device can be affixed to any otherdevice with blood flow. The analyzer is a computer-driven device and mayinclude an algorithm that analyzes intravascular pressure, hemodialysisvenous access pressure, and blood pump flow data to identify patientsat-risk for access dysfunction, either for thrombosis requiringpercutaneous transluminal angioplasty, or surgery to maintain accesspatency.

Alternatively, the device can be included as part of a hand-held device.In this embodiment, the device may replace the pressure gauge with ahand-held microprocessor controlled device that measures and records thepressure measurements. An algorithm in the device calculates the averagepressure over a predetermined sampling period. The device may alsocontain a computer database to recall individual patient information andto record current pressure measurements in the patient's databaserecord. Data from the device can be transferred via a communication portto a larger computer system with a more extensive patient database.

Generally, according to an aspect of the present invention, a method anddevice may be provided for monitoring and/or detecting failure in asystem based on pressure measurements. The present invention hasnumerous applications which can include, but is not limited to,mechanical, chemical, and biological arts. For instance, in chemicalprocesses, the present invention is useful where pressure changes areindicative of system failure. Additionally, the method and device of thepresent invention can be used for detecting any variation in bloodpressure and forwarding via the communicating device a warning regardingthis variation. The device and method therefore can be used in detectingpotential access failure, risk of stroke, risk of heart attack, risk ofstenosis, and risk of aneurysm.

More specifically, the present invention provides for a method ofdetecting a dislodged needle in a hemodialysis procedure by measuringvenous drip chamber pressure in a patient, analyzing the venous drippressure and deriving intravascular blood pressure at a location of thevenous needle insertion into the patient, comparing the derivedintravascular blood pressure to a standard which may have been developedfrom prior calculations during that particular session, and repeatingthe measuring, analyzing and deriving, and comparing steps to determineif the derived intravascular blood pressure is within a specified rangeof the standard, which may indicated that a needle has been dislodged inthe hemodialysis procedure. The steps of this method are generallydepicted in FIG. 10.

The venous drip chamber pressure (VDP) is the pressure that is actuallymeasured in the extracorporeal circuit (outside the body), and isfurther described below. The intravascular blood pressure is calculatedby analyzing the venous drip pressure and the deriving venous accesspressure (VAP) in proximity of a location of venous needle's point ofaccess on the body. These steps are further described below. The derivedintravascular blood pressure (VAP) is compared to a standard that can beset for the device or derived from prior measurements of VAP during thesession or from prior sessions for the patient as further describedbelow. Each of the measuring, analyzing and deriving, and comparingsteps may be repeated multiple times during the session when the medicaldevice is in use. More specifically, multiple VAP values are determinedover multiple time periods. It may be advantageous from a safety pointof view to make these measurements frequently.

Once the intravascular pressure (VAP) has been determined to be within aspecified range of the standard, possibly indicating that the needle hasbecome dislodged, an alarm circuit may be activated that thencommunicates via a communication device a command to alert the medicalstaff and/or turns off the blood pump of the hemodialysis machine, sothat the patient does not continue to lose blood.

The device may include an alarm that is activated and alerts medicalpersonnel to a problem with the patient. The alarm may provide a warningif the patient's needle came out of the access, i.e. became dislodged.Thus, the venous drip chamber pressure is equal to or close to venousdrip chamber at zero access pressure for an alarm to occur. Currently,dialysis machines cannot detect an opening of the venous return line andincidents of severe bleeding have been reported when the venous needlehas come out of the access site during dialysis. By detecting a drop inthe intravascular pressure of the patient, an alarm can be activated onthe detecting device that alerts medical personnel to the patient'scondition so that the needle can be replaced and the patient's life canbe saved from unnecessary blood loss. The alarm can also wake up thepatient if asleep so that the patient can alert medical personnel, andcan include a vibrating portion attached to the patient to assist inwaking up or alerting the patient.

The algorithm according to an aspect of the present invention calculatesthe actual pressure as seen at the tip of the needle by removing thepressure caused by the needle and tubing (VDP₀) from the measured VDP,which leaves VAP. By building the algorithm into the dialysis machine sothat VAP is calculated often, an alarm can be sounded when VAP drops tozero or near zero, thus indicating that the venous needle probably hasdislodged. This alarm determination can then a) turn off the machine sothat the patient does not lose more blood, and b) sound an alarm tonotify either the medical staff or the home care patient that a problemexists.

The algorithm in accordance with the present invention can be utilizedas an alarm system in any device that transports blood from a patient toan extracorporeal circuit and returns the blood to the patient. Thealgorithm determines the pressure at the point of insertion of the bloodinto the body based on a pressure reading in the extracorporeal bloodcircuit along with the rate of fluid flow through the device, thephysical properties of the fluid transported through the device and adetermination of the pressure inherent in the external circuit beginningfrom the pressure measuring device to the end of the needle at the pointof insertion into the body. The algorithm allows the alarm level to varywith the rate of fluid flow through the device. The present device canbe utilized as an alarm in plasmapheresis, heart lung machines and anyextracorporeal blood treatment or infusion technology circuits. Alarmsystems based on the present device are not limited to medicalapplications but can be developed for any fluid transporting device.Alarm levels can be set at any pressure value that provides safeoperation of the device.

The alarm can be a wireless alarm or a hardwired alarm. Morespecifically, a wireless alarm can send wireless signals to a handheldmonitor/device that is carried by medical personnel or to a centralmonitoring area, such as by the Internet or through communicationmechanisms that include, but are not limited to electroniccommunications, facsimile, telephone, cable modem, and T1 connection. Ahardwired alarm can send signals to any device that is in electricalconnection with the detecting device of the present invention, such as acentral monitoring area. The alarm can also be an audible warning orother similar signal that sends a command to the medical device (such asturn off) and/or wakes up the patient and alerts medical personnel.

Thus, by performing the method according to the present invention, if aneedle should become dislodged by the patient's movement during sleep orotherwise, the patient's life can be saved by turning off the machineand alerting medical personnel in time.

The invention further provides for a method of alerting medicalpersonnel of a dislodged needle in a hemodialysis procedure by detectinga drop in intravascular pressure derived from measured venous drippressure, detecting a dislodged needle, and alerting medical personnelof the dislodged needle. Each of the steps of this method is describedabove.

The detection device can be used to monitor any type of patient bloodaccess site for increased blood pressure and subsequently reduced bloodflow. The types of blood access sites that can be monitored include, butare not limited to, fistulas, grafts, catheters, or any type ofpermanent blood access port. In catheters and permanent blood accessports the plastic materials used to construct the devices become coatedwith layers of protein and fibrous substances that reduce the internaldiameter of the blood pathway or these devices may induce the formationof a vascular stenosis downstream of the implantation site. Anyreduction in internal diameter of the blood pathway that results in anincrease in pressure upstream of the catheter or permanent blood accessport can be detected by the algorithm in the present device and awarning can be issued once an appropriate alarm level is exceeded.

Additionally, the present invention can be applied to monitor thearterial line supplying the dialysis machine. A significant increase inthe negative pressure created by the dialysis machine blood pumpremoving blood from the patient can be used to indicate the presence ofan arterial stenosis or an obstruction of the arterial line. Further,the present invention can be utilized to describe the relationshipbetween blood flow, pressure, and hematocrit in any type of system thatremoves blood from a patient and returns the same blood to the patient.Thus, it can be used in conjunction with a heart-lung machine todetermine alarm parameters for blood withdrawal and reinfusion.

The detection device can be used with intravenous infusion systems todetermine the pressure profile for fluid infusion through a known tubingset and needle. A significant increase in the infusion pressure at thespecified fluid viscosity and flow rate can be used to determine alarmconditions and prevent infusion of fluid into the tissue if the needleis not inside the lumen of the vein. Further, any industrial system thatrequires regulation of infusion pressure can utilize the presentinvention to develop a monitoring system based on the analysis ofinfusion pressure.

Hemodialysis access monitoring programs that measure access flow orintra-access pressure have been developed for early detection ofevolving stenotic lesions (1-8). Studies have shown that early detectionof stenotic lesions followed by timely corrective procedures reduces thethrombosis rate and improves hemodialysis access survival (1, 3, 9, 10).Access monitoring programs are costly because they require equipment,personnel, data storage, and analysis. The method of the presentinvention includes an inexpensive technique known as the venous accesspressure ratio test (VAPRT), and obviates these encumbrances.

During hemodialysis, blood is drawn from the vascular access through thearterial needle by the hemodialysis machine blood pump. After passagethrough the dialyzer, the blood traverses the venous drip chamber andreturns to the access through the venous needle. The pressure requiredto infuse blood back into the access through the venous tubing andaccess needle and to overcome the pressure within the access is recordedas the venous drip chamber pressure (VDP). One component of VDP is theaccess pressure at the venous needle site (hereafter, termed “venousaccess pressure” (VAP)). Another component of VDP is the combinedpressure required to overcome the resistance to flow through the tubingdistal to the drip chamber (low) and through the venous return needle(high). VDP is also a function of needle size, tubing length and bloodviscosity, represented by hematocrit. If the venous pressure within anaccess at the needle site is 0 mmHg, VDP can be defined as VDP₀, i.e.,the venous drip chamber pressure when the access pressure is zero.Consequently, VDP₀ can be calculated for a given hemodialysis machine,tubing set, and needle size when the blood flow rate and hematocrit aremeasured. Once VDP₀ is determined, VAP can be calculated from themeasured VDP.

VAP=VDP−VDP₀  Equation (1)

An elevation of VAP indicates stenosis in the venous outflow of theaccess and is associated with increased access failure probability (6,8, 11, 14). To normalize variations in VAP attributed to changes in meanarterial pressure (MAP), the venous access pressure ratio (VAPR) iscalculated by dividing VAP by MAP.

VAPR=VAP/MAP  Equation (2)

The data that yields the determination of VDP₀ is contained within acentral database repository that holds dialysis laboratory data andparameters acquired from hemodialysis machines that directly communicatewith computers in the dialysis units. The VAPRT algorithm utilizes anempirical formula to calculate VAP from a dynamic measurement of VDPobtained at treatment and digitally recorded. The VAPRT algorithmanalyzes monthly VAPR values and identifies individuals withconsistently elevated intra-access pressures at risk for access failure.To eliminate treatment errors such as needle reversal or suboptimalneedle placement that cause elevated VDP, an abnormal VAPRT wasoperationally defined as VAPR>0.55 at three treatments.

Analysis of the data for the hemodialysis machine circuit yielded thefollowing second order polynomial equation, henceforth referred to asEquation (3):

VDP₀=0.00042*Qb ²+(0.62116*Hct²+0.01203*Hct+0.12754)Qb−17.32509  Equation (3)

Equation (3) can be used to calculate VDP₀ for any Qb at known Hct. Forexample, at Qb=500 ml/min and Hct 18.2%, VDP₀ is 163 mmHg and increasesto 200 mmHg when Hct=38.4%. VAP can be calculated from VDP recorded atHD by Equation (1) and VAPR is calculated by Equation (2). At Hct 38.4%,Qb 500 ml/min, VDP 265 mmHg, VDP₀ 200 mmHg, and MAP 100 mmHg,VAPR=0.65=(265−200)/100. In the case where blood flow (Qb) is equal tozero in Equation (3), the following occurs:

VDP₀=0+0−17.32509=17.32509

Venous access pressure (VAP) is then calculated using Equation (1).

VAP=VDP−VDP₀ VAP=VDP−(−17.32509) VAP=VDP+17.32509

The constant (−17.32509) is determined by the dialysis machine type andthe level of the patient's access site. Clinical studies have shown thatthe venous drip chamber pressure recorded by the machine and correctedfor the height difference between the drip chamber transducer thepatient's access gives an accurate value for venous access pressure (8,22). The algorithm can therefore be incorporated into the dialysismachine. The dialysis machine therefore automatically records thereadings. Additionally, a sensor can be placed on the hemodialysismachine to determine the height difference between the venous dripchamber transducer and the level of the patient's access site.

The VAPRT relies on a nonlinear regression formula to calculate VDP₀ forspecific hemodialysis blood tubing set and access needle when thepatient's hemodialysis blood pump flow (Qb) and hematocrit are known.The formula was developed from data analysis obtained during in vitrosham hemodialysis. FIG. 1 shows a diagram of the experimentalhemodialysis system. The dialysis machine (Fresenius 2008H, Lexington,Mass., U.S.A.) blood pump was calibrated prior to experiments using thestandard maintenance procedure. The exact flow was not measured duringthe in vitro experiment as the intention a priori was to design amonitoring system that utilized routine dialysis data obtained from eachdialysis treatment. The reservoir is filled with 500 ml of human wholeblood obtained from the hospital blood bank. The blood pump transportsblood from a reservoir through the dialyzer and the venous drip chamberand then to a 15 gauge, 1-inch backeye access needle. The venous accessneedle is inserted into a section of large-bore tubing that is open atboth ends. One end of the tubing returns blood to the reservoir and theother end is elevated to prevent blood from escaping. This section ofthe circuit is not designed to simulate an actual access, but to avoidany resistance to flow at the tip of the venous access needle that canbe recorded as an increase in VDP. The access needle is positioned 17 cmbelow the venous drip chamber transducer to simulate the averagelocation of an angioaccess relative to the transducer during a typicalhemodialysis treatment. The drip chamber transducer monitors thepressure created by the blood flowing through the circuit. VDP₀ readingsare obtained directly from the hemodialysis machine. A sample of bloodis obtained for hematocrit determination from the reservoir. VDP₀ isrecorded as Qb is increased from 0 to 600 ml/mm in 50 ml/mm increments.A separate transducer, placed directly behind the access needle,measures the pressure created by the access needle's intrinsicresistance. The blood is then diluted with matched human plasma to lowerhematocrit by approximately 4%. Blood is permitted to circulate at 500ml/mm for 5 minutes to ensure uniform mixing with the additional plasmabefore the next sample is obtained for hematocrit measurement. VDP₀measurements are repeated for Qb from 0 to 600 ml/mm. The circulatedblood is diluted five times, reducing the original hematocrit byapproximately 20 percentage points. VDP₀ measurements were conducted ateach of the five dilutions.

The test monitors for a persistent elevation of the VAPR to identify anaccess that requires additional evaluation. The algorithm calculatesVAPR from VDP and blood pump flow data that is routinely collectedduring hemodialysis and stored in a computer database. The algorithmdetermines whether a persistent increase in VAPR is present duringsequential treatments.

To limit variability intrinsic to differences in needle gauge, patientswith less than 48 hemodialysis treatments were eliminated from analysisbecause a smaller gauge needle is frequently used when initiallycannulating a new or poorly developed angioaccess. The program extractsthe most recent hematocrit and individual treatment data from thecomputer database and analyzes data for those patients who receivetreatments via a graft. The VAPR is calculated each time the bloodpressure is measured during hemodialysis, given the following criteria:Qb≧200 ml/mm, VDP≧20 mmHg and MAP≧75 mmHg. Data from the last hour ofhemodialysis is excluded to eliminate the effect of ultrafiltration onhematocrit (elevated blood viscosity), blood pressure, and changes insystemic and vascular access resistances. The algorithm then calculatesthe mean VAPR for each hemodialysis treatment using all available data.In the majority of cases three or four measurements are available.Patients with <10 hemodialysis treatments during a month were excluded.The VAPRT is considered positive when, starting with the eighthtreatment of the month; the program determines that the VAPR exceeds thespecified cutoff value during three consecutive treatments.

The invention is further described in detail by reference to thefollowing experimental examples. These examples are provided for thepurpose of illustration only, and are not intended to be limiting unlessotherwise specified. Thus, the invention should in no way be construedas being limited to the following examples, but rather, should beconstrued to encompass any and all variations that become evident as aresult of the teaching provided herein.

EXAMPLES Example 1 Criterion for the Venous Access Pressure Ratio Test

To determine the VAPR cutoff value most predictive of access failure,test data and follow up data were analyzed from 117 patients with graftswho received hemodialysis treatment at three hemodialysis facilitiesduring January 1999. VAPR in these patients were correlated with thepresence or development of access dysfunction, stenosis requiringintervention by angioplasty or surgical revision to maintain accesspatency, or the occurrence of thrombosis within the six months of followup observation. A six month observation period was selected because datareported showed that primary unassisted patency for grafts at six monthsis 64% and secondary assisted patency is 70% at six months, which is inaccordance with data from Sparks (15) showing a primary patency forgrafts of 64% at a median of seven months. The data from these studiesindicates that in any six month period 30 to 36% of all grafts can fail.The VAPRT is being used to try and identify grafts in this group beforethey fail.

A receiver operator curve (ROC) for VAPRT was constructed with cutoffratios of 0.2, 0.3, 0.4, 0.45, 0.5, 0.55, 0.6 and 0.8 while other testparameters were held constant. The respective sensitivities andspecificities were calculated at each VAPR cutoff level. Areas under thereceiver operator (ROC) curves were calculated using Mathcad Plus 6.0(MathSoft Inc., Cambridge, Mass., U.S.A.). Clinical results wereanalyzed with StatView for Windows v. 5.0 (SAS Institute, Inc., Cary,N.C., U.S.A.) and DeltaGraph 4.0 (SPSS, Inc., Chicago, Ill., U.S.A.).Grouping variables for unpaired t-tests were true positive (TP; testpredicted intervention or access clotting), true negative (TN; testcorrectly predicted the absence of an access event), false positive (FP;test falsely predicted an access event would have occurred) and falsenegative (FN; test falsely predicted that an access event would notoccur). The hypothesized difference between groups for all comparisonswas zero.

Clinical Application of Venous Access Pressure Ratio Test

A total of 359 VAPRT were acquired from ESRD patients in threeGreenfield Health System hemodialysis units over a three month intervalfollowing the determination of the optimal VAPR=0.55. The samepopulation's data was retrospectively analyzed from January (n=112),February (n=113) and March (n=134) of 1999. Medical records wereexamined to identify those individuals who required intervention for anaccess event, defined as an obviously low access flow (<250 ml/mm), aninability to provide adequate dialysis within the predeterminedtreatment time or surgical or angioplasty intervention to maintainaccess patency, from stenosis or thrombosis.

Results

In vitro Modeling of VAP₀

Derivation of the Mathematical Model

Results of the sham dialysis study are shown in FIG. 2. Mathematicalmodeling of VDP₀ data is shown in FIG. 2. The data in FIG. 2 wasanalyzed by fitting each individual curve with an equation of the form:

VDP₀ =A*Qb ² +B*Qb+C  Equation (1a)

The constant C represents the value of VDP when Qb=0 and the averagevalue of −17.325 mmHg was used during further analysis of the data.Because coefficient A varied minimally from 0.0004232 to 0.0004327, anincrease of only 1.5 mmHg in VDPQ at Qb=400, a mean value of 0.00042329was used. Coefficient B varied the most with hematocrit from 0.145289 to0.231968. The raw data was then fit with Equation (2a).

VDP₀=0.00042329*Qb ² +B*Qb−17.325  Equation (2a)

B coefficients were obtained for each hematocrit value. FIG. 6 displaysthe plot of Coefficient B versus hematocrit and Equation (3a) was fit tothe data.

B=0.62116*Hct ²+0.01203*Hct+0.12754  Equation (3a)

Equations (2a) and (3a) were combined to yield Equation (4a) thatrelates VDP₀ to Qb and Hct.

VDP₀=0.00042*Qb ²+(0.62116*Hct²+0.01203*Hct+0.12754)*Qb−17.32509  Equation (4a)

Equation (4a) was evaluated for accuracy using a nonlinear regressionprogram (DataFit, Oakdale Engineering, Oakdale, Pa., U.S.A.). Theadjusted coefficient of multiple determination r²=0.99982 validated thatEquation (4a) represents an accurate mathematical model of the pressuredata for access monitoring by dynamic VAPRT.

Application of the Mathematical Model

Analysis of the experimental data for the hemodialysis machine circuityielded the following second order polynomial equation, henceforthreferred to as Equation (3):

VDP₀=0.00042*Qb ²+(0.62116*Hct²+0.01203*Hct+0.12754)*Qb−17.32509  Equation (3)

The common average intercept, −17.35, was established empirically and isrelated to the 17 cm difference in height between the needle and dripchamber transducer at Qb=0. When pressure is measured from thetransducer proximal to needle, the offset becomes zero, and therelationship between pressure and flow remains curvilinear (FIG. 2,venous needle pressure at Hct=29.1). Thus, VDP₀ increases inrelationship to increasing Qb and hematocrit.

Equation (3) can be used to calculate VDP₀ for any Qb at known Hct. Forexample, at Qb=500 ml/min and Hct 18.2%, VDP₀ is 163 mmHg and increasesto 200 mmHg when Hct=38.4%. VAP can be calculated from VDP recorded atHD by Equation (1) and VAPR is calculated by Equation (2). At Hct 38.4%,Qb 500 ml/min, VDP 265 mmHg, VDP₀ 200 mmHg, and MAP 100 mmHg,VAPR=0.65=(265−200)/100. In the case where blood flow (Qb) is equal tozero in Equation (3), the following occurs:

VDP₀=0.00042*Qb ²+(0.62116*Hct+0.01203*Hct+0.12754)*Qb−17.32509

When Qb=0 venous access pressure (VAP) is then calculated using Equation(1).

VDP₀=0+0−17.32509=−17.32509

VAP=VDP_VDP₀ VAP=VDP−(−17.32509) VAP=VDP+17.32509

The constant −17.32509 is determined by the dialysis machine type andthe height of the patient's access site. Clinical studies have shownthat the venous drip chamber pressure recorded by the machine andcorrected for the height difference between the drip chamber transducerthe patient's access gives an accurate value for venous access pressure.The algorithm can therefore be incorporated into the dialysis machine.The dialysis machine therefore can automatically take the readings.Additionally, a sensor can be placed on the machine to determine theheight difference between the venous drip chamber transducer and thelevel of the patient's access site.

Receiver Operator Curve (ROC) Evaluation

Patients with grafts (N=117) included during the January 1999 testperiod and whose data were used for ROC analysis had mean treatmentblood flows 438±61 ml/mm, hematocrit 34.0±4.2% MAP 102±14 mmHg, VDPvalues ranging from 48 to 430 mmHg (mean 214±43 mmHg), and mean VAPR0.64±0.35.

The receiver operator curve (ROC) is shown in FIG. 3. The area under thecurve corresponds to the probability (0.82) of correctly ranking the twotest alternatives, persistence of access patency or occurrence of accessfailure within six months (16, 17). The VAPR cutoff of 0.55 was selectedfor further clinical testing as it provided a rational compromisebetween sensitivity (75%) and specificity (83%).

FIG. 4 shows the distribution of individual treatment mean VAPR valuesfor all patient observations with grafts in January 1999. The monthlymean VAPR for each patient was calculated from the VAPR values obtainedat each treatment. Patients who had a TP test by VAPRT had a median VAPR0.89 (mean 0.91±0.24). This value was significantly different from theother three possibilities, FP, TN, and FN (Table 1). Patients with TNtests had a median VAPR of 0.48 (mean 0.52±0.15), which differed from FP(median VAPR 0.70, mean 0.70±0.13 P<0.0001) but not from FN (median VAPR0.57, mean 0.62±0.23). All test groups had VAPR values greater than 1.0,in this case VDP-VDP₀ exceeds the mean arterial pressure for the dataobtained during treatment and can indicate a problem with needleplacement or needle reversal.

Assessment of the VAPRT

FIG. 5 shows the study results of three months of VAPRT for January,February, and March of 1999. In January 26 out of 112 patients (23%) hada positive VAPRT. During the next three months, thirteen of thesepatients (50%) experienced access failure, by month six the numberincreased to nineteen (73%) in the positive test group. For the Januarytest, eight patients that tested negative went on to experience accessfailure (FN, 7% of population tested). The statistical analysis of theVAPRT are shown in Table 2 and represent the average at three and sixmonths after each test. For the three month follow-up period, the meantest sensitivity of VAPRT was 70±8% while the specificity was 88±2%.These improved to a mean sensitivity of 74±5% and specificity of 96±3%for the six month follow-up period. The VAPRT positive predictive valuewas 84±10% and the negative predictive value 92±3% for the six monthfollow-up period.

Discussion

The location of an access stenosis, in part, determines the ability of amonitoring system to detect the lesion. In most grafts, a stenoticlesion develops in the region of the venous anastomosis (10, 11, 12,13). A stenosis in this region or in the central vein impedes blood flowthrough the access and increase VAP, which is observed as an increase inVDP. VDP measured during treatment is the sum of three components; thepressure created by blood flowing through the tubing and the needle(VAP₀), the static pressure created by the difference in height betweenthe access site and the venous pressure transducer in the dialysismachine and VAP. VDP varies with treatment Qb, VAP, and hematocrit. Thedifference in height between the access site and the venous pressuretransducer also varies, but, in most cases, does not differ by more than5 cm from the value of 17 cm used in the model. This results in a ±5.1mmHg variation in VAP and at MAP=100 mmHg a ±0.05 variation in VAPR. VAPalso varies with the MAP and changes in MAP are reflected in VDP.Mapping of the access pressure gradient from the arterial to the venousanastomosis has shown that the slope of the mid graft pressure gradientincreases with the development of a stenosis (11). Therefore, VDPincreases with increasing distance between the venous needle and venousanastomosis.

Initially it appears that values of VAPR exceeding 1.0 are biologicallyimpossible; however, all tests groups had some VAPR values>1.0,reflecting that physiologically calculated VAP exceeded MAP. For theVAPR data presented in FIG. 4, 9.8% of all values were >1.0, with 27.9%of these in the TP group. Several conditions lead to higher thanexpected VAPR values. Reversal of arterial and venous needles isprobably the most common and occurs in as many as 25% of treatments(18). If a smaller diameter needle is used, without indicating thechange in the patient's treatment data, the VAPR values will be falselyelevated. It can also be noted that the small diameter of the venousneedle creates turbulent flow in the access that increases resistance toflow through the access. The degree of turbulent flow increases whenaccess flow is reduced due to a venous stenosis and results in increasedflow resistance and increased VAP. Lodgment of the venous needle againstor partially in the access wall (reduces the needle orifice) or a venousline obstruction produces an increase in the measured VDP and results inepisodic high VAPR values. Finally, a difference in MAP in the accessextremity from that of the non-access arm that is typically used tomonitor blood pressure during hemodialysis (19), which results in anincrease in VAPR.

To reduce errors in the VAPRT, patient VAPR values must exceed 0.55 forthree consecutive treatments. Initial dynamic access pressure testingdeveloped by Schwab used three consecutive treatments that exceededpredefined limits to indicate a positive test. Dialysis treatments atthe end of the month were selected for evaluation because the testresults were included in a monthly dialysis patient report and patientsmay have had an access intervention during the early part of the month.The objective was to maintain a minimal false positive rate to preventunnecessary further evaluation of the patient's access.

FIG. 2 illustrates the problems that must be resolved when using dynamicmeasurements of VDP to monitor access pressure. As blood flow increasesVDP increases, primarily attributed to augmented resistance created bythe venous needle. Elevation of hematocrit also increases VDP. Thevariability in VDP values from Qb and hematocrit can be reduced if themeasurements are made at a fixed, relatively low, blood flow, asdemonstrated by Schwab et al (1). However, the appropriate warning levelfor VDP varies among individuals depending on the MAP and hematocrit.For example, with a 15 gauge needle and Qb=200 ml/mm, VDPQ is 33 mmHg athematocrit 20% and 42 mmHg at hematocrit 36%. Using the criteria that apatient is at risk when the access pressure ratio>0.55, a patient with aMAP of 120 mmHg requires an access pressure >66 mmHg (66/120=0.55) toreceive a warning for that treatment. Therefore at Qb=200 ml/mm, the VDPwarning level is between 99 (=33+66) mmHg and 108 (=42+66) mmHg for apatient when hematocrit varies between 20% and 36%. Applying the samecriteria, a patient with MAP=75 mmHg needs a VDP warning level between74 and 83 mmHg. It then becomes difficult to select a single VDP warningvalue for patients at risk for VDP between 74 and 108 mm Hg. By usingEquation (2) to calculate VAPR, the VAPRT adjusts the VDP warning levelfor each access pressure measurement in relationship to Qb, hematocritand MAP. Notably, this absolute pressure range of 74 to 108 mm Hg issignificantly lower than that originally reported by Schwab et al (1).The major reason for this difference is needle gauge, 15 gauge for thepresent invention versus 16 gauge for the Schwab investigation. Thecomponent of VDP due to flow through the needle is expected to besignificantly greater with a 16 gauge needle (6). Presently, thealgorithm is limited to 1 inch 15 gauge needles for cannulation untilinvestigation of other needle gauges has been carried out.

An alternative method of determining the VAPR is to monitor staticvenous pressures and calculate a static venous access pressure ratio(SVPR) to test for a functionally significant stenosis (8). SVPR is anaccurate method for access monitoring, however this method involvestraining of hemodialysis staff and ongoing monitoring to ensure thevalidity of the data. The VAPRT does not require specific training andthe algorithm examines data currently entered in the patient databaseand evaluates the patient's access for each dialysis treatment. Finallyanother method measures static intra-access pressures directly prior tohemodialysis using a hydrophobic filter (22).

A stenosis on the arterial input side of the access or within the accessitself is not detected by the VAPRT because this type of lesion reducesaccess flow and venous access pressure simultaneously. It is feasible todetect an arterial stenosis by developing a model that examines pre-pumparterial drip chamber pressure (ADP) for values more negative thanusual. It is also possible to determine the existence of intra-accesslesions if arterial intra-access pressure and VAP can be determined. Inthis regard, Polaschegg and colleagues (20) described a method fordetecting and locating an access stenosis using dynamic arterial andvenous access pressure measurements.

Access flow measurements performed within the dialysis unit candetermine whether there is a clinically significant reduction of accessflow, indicating the necessity for intervention. However, the locationof the flow obstruction cannot be definitively identified. Thedisadvantages of flow measurements are that they require costlyequipment, trained personnel and dialysis time for setup andmeasurement. Studies by Paulson et al. (17, 21) indicate that a singleaccess flow measurement is a relatively poor indicator of graft failure.To achieve a sensitivity of 80% for predicting thrombosis requires anunacceptably high FP rate of 58%. The FP rate is so high because thethreshold access blood flows that are used to predict graft failureoften include many grafts that function at low blood flows, on the otherhand, some grafts with good flows inexplicably thrombose without anywarning.

Analysis of the data demonstrated that at a sensitivity of 80% the FPrate was 34% for testing grafts. A low FP rate (20% for grafts) wasmaintained in order not to produce a large number of evaluations thatresults in interventions by either vascular surgeons or interventionalradiologists. It has been suggested that trend analysis can be a betterpredictor of access failure when using access flow. Trend analysisrequires more frequent flow measurements and greatly increases the costof access flow measurements. The VAPRT calculates a VAPR for eachdialysis treatment, rendering it ideal for trend analysis. The currentVAPRT models the VAPR trend after the eighth treatment of a month. Tominimize spurious alarms, a triplet rule was imposed whereby threeconsecutive treatments with VAPR>0.55 were necessary to elicit a warningof impending graft failure, and this rule is currently being applied togenerate an end-of-month report to assist clinicians in identifyingpatients with grafts at risk for dysfunction. It is possible to improvethe VAPRT test if trend analysis of the all data is included in thealgorithm. Greater emphasis can be placed on temporal trends or datafilters imposed to exclude clearly erroneous measurements. In addition,analysis of data from two or more consecutive months can increase thepower to detect access dysfunction.

The results of this study demonstrate that the VAPRT is a usefulnoninvasive screening test that identifies a population of dialysispatients that is at risk for access failure. The key component inimplementing this system is computer access to the required treatmentand laboratory data. The software algorithm to analyze hemodialysis datais incorporated as a standard end-of-month report and as anInternet-based accessible vascular access monitoring system. Allpatients exhibiting a warning status are flagged and a database triggeris available on-demand to create a report for any location or timeperiod. Access intervention can be tracked along with warning status,thus permitting immediate follow-up and timely cost-savinginterventions.

Example 2

An alternative method is provided for measuring access pressure throughan access needle that is flow-connected to the vascular system of apatient. The method comprises the steps of: connecting one end ofpressure tubing to the outer end of the access needle tubing, with amembrane blocking the flow of blood while permitting the passage of airthrough to a pressure gauge. The membrane suppresses or dampens thepressure pulses or oscillations through the tubing. Thus, upon openingthe access needle tubing to the vascular system, blood flowing into thetubing compresses the air in the pressure tubing, plus the connectedgauge, causing pressure from the vascular system to be readable by thegauge while the pressure pulses are attenuated in a simple,nonelectronic manner.

The “membrane” mentioned above may be a microporous membrane, typicallya microporous block or plug positioned within or adjacent to thepressure tubing and capable of providing the damping or attenuation ofthe pulsatile nature of the pressure from the patient's cardiovascularsystem at the gauge.

According to an aspect of the present invention, the internal volume ofthe pressure tubing is less than the internal volume of the accessneedle tubing. As the result of this, pressurized blood entering theempty access needle tubing as the pressure is read does not advancecompletely to the level of the membrane, but is halted by compression ofthe initial air in the tubing, as well as the residual volume of airwithin the pressure gauge. This can be accomplished by providingpressure tubing that has a connector at each end, the tubing having asingle lumen of reduced diameter from normal flexible tubing, whichlumen diameter is typically no more than about one third of the outerdiameter of the tubing. Thus, the internal volume of the pressure tubingcan be less than the internal volume of the first tube even if thelength of the pressure tubing is greater than the length of the firsttube, this situation is preferred so that there is adequate tube lengthto conveniently hold a pressure gauge and to position it atapproximately the level of the patient's heart and to read it with ease,and also to reduce the chance that the access needle connection to thepatient's access is disturbed as the pressure gauge is connected andhandled.

The set that defines the pressure tubing may carry a microporous memberthat is capable of preventing the passage of bacteria therethrough. Thiscan be a second microporous member if desired, above and beyond themicroporous plug described above that suppresses pressure oscillationsthrough the pressure tubing, thus attenuating the pressure pulses. Aconventional 0.2 micron bacterial filter can be used. This uniquelyprovides both flow blocking and aseptic conditions with commerciallyavailable materials.

Alternatively, the microporous member can be a plug that has a bacteriablocking capability similar to conventional 0.2 micron bacterialfilters. Also, a membrane-type bacterial filter can have pores that aresmall enough to provide the desired attenuation of pressure pulsesthrough the pressure tubing, to facilitate reading of the gauge.

Also, if desired, the pressure tubing can have a bore that issufficiently narrow and of a length to provide the desired pressurepulse attenuation through the tubing without the need for a porous plugso that, typically, only a bacteria blocking filter membrane isprovided, as needed, to protect the patient from bacterial contaminationthrough connection to a nonsterile pressure gauge.

Further development of the device includes replacement of the pressuregauge with a handheld microprocessor controlled device that measures andrecords the pressure measurements. An algorithm in the device calculatesthe average pressure over a predetermined sampling period. The devicealso contains a computer database to recall individual patientinformation and to record current pressure measurements in the patient'sdatabase record. Data from the device can be transferred via acommunication port to a larger computer system with a more extensivepatient database.

Example 3

This example demonstrates the case where blood flow (Qb) is equal tozero in Equation (3). The constant term (−17.32509 in Equation (3))needed to correct for the difference in height between the venous dripchamber and the level of the patient's access site was calculated forthree different dialysis machines and clinical data was evaluated todemonstrate the effectiveness of the system.

The measurement of venous intra-access pressure (VAP) normalized by meanarterial blood pressure (MAP) facilitates detect venous outlet stenosisand correlates with access blood flow. General use of VAP/MAP is limitedby time and special equipment costs. Bernoulli's equation relatesdifferences between VAP (recorded by an external transducer as PT) andthe venous drip chamber pressure (VDP) at zero blood pump flow, thedifference in height (ΔH) between the measuring sites and fluid densitydetermine the pressure due to the difference in height ΔPH-VAP-VDP. Theywere therefore correlated VDP and PT measurements at six differentdialysis units each using one of three different dialysis machines. Bothdynamic (i.e. with blood flow) pressures and static pressures weremeasured. Validation studies showed that changes in mean blood pressure,zero calibration errors, and hydrostatic height between the transducerand drip chamber accounted for 90% of the variance in VDP withΔPH=−1.6+0.74*ΔH (r=0.88, p<0.001). The major determinant of staticVAP/MAP was access type and venous outflow problems. In grafts, flowaveraged 555±45 mL/min for VAP/MAP>0.5 and 1229±112 mL/min forVAP/MAP<0.5. ΔPH varied from 9.4 to 17.4 mm Hg among the six centers andwas related to ΔH between the drip chamber and the arm rest of thedialysis chair. Concordance between the values of VAP/MAP calculatedfrom PT and from VDP+PH was excellent. It was concluded that static VDPmeasurements corrected by an appropriate ΔPH can be used toprospectively monitor prosthetic bridge grafts for stenosis.

Throughout this application, various publications, including UnitedStates patents, are referenced by author and year and patents by number.Full citations for the publications are listed below. The disclosures ofthese publications and patents in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the state of the art to which this invention pertains.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. It is understood that the features ofvarious implementing aspects may be combined to form further aspects ofthe invention. The words used in the specification are words ofdescription rather than limitation, and it is understood that variouschanges may be made without departing from the spirit and scope of theinvention.

TABLE 1 Comparison of Monthly Mean Graft VAPR Values for the DifferentTest Groups Count Mean Std. Dev. Std. Err True Positive 27 0.909 0.2370.046 True Negative 67 0.515 0.149 0.018 False Negative 9 0.616 0.2150.072 False Positive 14 0.698 0.125 0.033 Mean Difference p-Value TruePositive, True Negative 0.394 <0.0001 True Positive, False Negative0.293 0.0024 True Positive, False Positive 0.211 0.0036 True Negative,False Positive −0.183 <0.0001 True Negative, False Negative −0.1020.0734 False Positive, False Negative 0.082 0.2595

TABLE 2 Statistical Analysis of Venous Access Pressure Ratio Test forGrafts Showing Mean Values for Three Months of Testing Test Period 0-3mo 0-6 mo Sensitivity (%) 70 ± 8 74 ± 5 Specificity (%) 88 ± 2 96 ± 3Positive Predictive Value (%)  52 ± 10  84 ± 10 Negative PredictiveValue (%) 94 ± 2 92 ± 3 False Positive rate (%) 12 ± 2  4 ± 3

REFERENCES

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1. A method of detecting a dislodged needle in a hemodialysis procedure, comprising the steps of: measuring venous drip pressure for a patient undergoing dialysis; analyzing the venous drip pressure and deriving intravascular blood pressure at a location of venous needle insertion into the patient; comparing the derived intravascular blood pressure to a standard; repeating said measuring, analyzing and deriving, and comparing steps; and if the intravascular blood pressure is within a specified range of the standard, determining that a needle has been dislodged in the hemodialysis procedure.
 2. The method of claim 1, further including the step of turning off the blood pump of hemodialysis machine.
 3. The method of claim 1, further including the step of activating an alarm that notifies medical personnel of the dislodged needle.
 4. The method of claim 3, wherein the alarm is chosen from the group consisting of wireless and hardwired.
 5. The method of claim 3, wherein the alarm is wireless and sends a signal to a handheld device of a medical personnel.
 6. The method of claim 3, wherein the alarm is audible.
 7. The method of claim 3, wherein the alarm vibrates and is connected to an alarm device located on the patient.
 8. The method of claim 1, wherein said analyzing step is further defined as deriving venous access pressure (VAP) in proximity of a location of needle insertion into the patient's body.
 9. The method of claim 8, wherein said repeating step is further defined as determining multiple VAP values over multiple time periods.
 10. The method of claim 8, wherein said comparing step is further defined as comparing the derived VAP to a standard.
 11. A method of alerting medical personnel of a dislodged needle in a hemodialysis procedure, comprising the steps of: detecting a drop in intravascular pressure derived from measured venous drip pressure; detecting a dislodged needle; and alerting medical personnel of the dislodged needle.
 12. The method of claim 11, further including the step of initiating a state for the hemodialysis machine that is safest for the patient, stopping the blood pump and initiating alarm systems to warn the patient and alert medical personnel.
 13. The method of claim 11, wherein the hemodialysis procedure is performed in a device that transports blood from a patient to an extracorporeal circuit and returns the blood to the patient. 